Microstructure and Mechanical Properties of Co32Cr28Ni32.94Al4.06Ti3 High-Entropy Alloy

High-entropy alloys have good application prospects in nuclear power plants due to their excellent mechanical properties and radiation resistance. In this paper, the microstructure of the Co32Cr28Ni32.94Al4.06Ti3 high-entropy alloy was researched using metallurgical microscopy, X-ray diffraction, and scanning electron microscopy. The mechanical properties were tested using a Vickers microhardness tester and a tensile testing machine, respectively. The results showed that Co32Cr28Ni32.94Al4.06Ti3 had a single-phase, disordered, face-centered, cubic solid-solution structure and was strengthened by solid solution. The alloy lattice parameter and density were estimated as 0.304 nm and 7.89 g/cm3, respectively. The test results indicated that the alloy had satisfactory mechanical properties with yield stress and tensile strength of about 530 MPa and 985 MPa, respectively.


Introduction
High-entropy alloys (HEAs) are a type of alloys which contain five or more principle alloying elements to stabilize solid solution phases by maximizing configurational entropy. HEAs have been extensively studied since they were first reported in 2004 [1]. The novel design idea underlying HEAs has greatly expanded the composition range and research areas of metallic materials. Owing to the high entropy effect, the alloys tend to form simple microstructures, such as body-centered cubic (BCC), face-centered cubic (FCC), and densely packed hexagonal (HCP) [2,3]. It has been found that some HEAs not only have excellent mechanical properties [4,5], but also excellent thermal stability [5], wear resistance [4][5][6][7], and irradiation resistance. Therefore, HEAs are promising materials for application in nuclear power plants.
The phase composition and mechanical properties of HEAs are determined by the element types and their atomic ratios in the alloy [8]. For transition HEAs, AlCoCrFeNiZr x is known to possess a BCC structure when x = 0 [9]; with an increase in the Zr content, the mechanical properties are significantly improved, because the phase composition of the alloy changes from an ordered BCC solid-solution phase to an ordered Laves + BCC phase. The phase composition of the AlCoCrFeNiB x alloy changes from a BCC phase to BCC + FCC two-phase solid solution with an increase in the B content; meanwhile, the hardness and fracture strength increase first and then decrease [10]. CoCrFeNiW x is known to possess a single-phase structure for x = 0.2 [11]; the phase composition of the alloy changes from a single-phase to a hypoeutectic phase, and the mechanical properties are significantly improved when the W content is increased. The phase composition of Al x CoCrFeNiTi changes from α + Al grains to fine equiaxed crystals when the Al content is increased; in this way, the yield stress is increased, but the elongation is decreased [12].
AlFeCrCoNi only exhibits an FCC structure for a low ratio of Al/Ni. However, for a large ratio of Al/Ni, the alloy exhibits a BCC structure. The hardness increases with the increase in the ratio of Al/Ni [13].
The three-component alloy of CoCrNi has a single FCC structure. Its mechanical properties are better than other alloys in the alloy system, especially at liquid nitrogen temperature [14][15][16], but the strength is relatively low at chamber temperature [16]. The alloy used in this study is a new, unreported alloy with Al and Ti elements added. Therefore, in this paper, the microstructure and tensile properties of Co 32 Cr 28 Ni 32 . 94 Al 4 . 06 Ti 3 were researched in order to better understand the microstructure and mechanical properties of CoCrNi HEAs.

Experimental Materials and Methods
A Co 32 Cr 28 Ni 32 . 94 Al 4 . 06 Ti 3 plate with dimensions of 90 mm × 80 mm × 20 mm was prepared using vacuum arc melting equipment with pure metal Co, Cr, Ni, Al, and Ti with purities greater than 99.9 wt%. In order to ensure that the material was not oxidized in the melting process, pure metal Ti was melted firstly to absorb the residual gas of the electric arc furnace. The composition (atomic fraction and weight ratio, %) of the Co 32 Cr 28 Ni 32 . 94 Al 4 . 06 Ti 3 used in this study is shown in Table 1. The crystal structure was identified using a DY1602/Empyrean multifunctional X-ray polycrystalline diffractometer (PAnalytical, Alemlo, Holland) with Cu target radiation scanning in the range from 20 • to 90 • at a rate of 2 • /min in a working voltage of 40 kV, a working current of 100 mA, and a characteristic wavelength of 1.5406 Å. The angle measurement accuracy was 0.02 • . The HEA density was measured using a drainage method. In the drainage method, the mass of the HEA was measured by a scale; a measuring cylinder was used to measure the volume of water, and the volume of the HEA was the changed volume of the water in the cylinder before and after the HEA was put into the cylinder. The metallography of the HEA was characterized using a MV5000 metallographic microscope (Nanjing Lianchuang Analytical Instrument Manufacturing, Nanjing, China). The Vickers hardness of the HEA was measured using a THV-1MD microhardness tester (Teshi Detection Technology, Shanghai, China) under a load of 200 gf applied for 15 s. The tensile specimens with dimensions of 51.83 mm × 14 mm × 2.5 mm were used, as shown in Figure 1. Three tensile tests were carried out at air room temperature with a strain rate of 1 × 10 −3 mm/s. The fracture morphology of the tensile specimen was analyzed using a Mira3 ultra-high-resolution field emission scanning electron microscope (FE-SEM) (TESCAN, Shanghai, China).    Generally, Ω > 1.1 and δ < 6.5% are the criteria for determining whether a solid solution phase can be formed [13]. Furthermore, when valence electron concentration (VEC) ≥ 8, the FCC solid solution is considered to be relatively stable [17]. The relevant parameters are calculated as follows:

Results and Discussion
where ∆H mix is the total enthalpy of mixing for the system, ∆H mix ij is the mixing enthalpy of the atomic pair of the ith and jth atoms, c i and c j are the atomic percentages of elements i and j, respectively, δ is the difference in atomic radius between the two atoms, r i is the atomic radius of the ith element, VEC is the total VEC of the system, (VEC) i is the VEC of the ith element, Ω is the disorder of the system, T m is the mixed melting point of the system, ∆S mix is the mixing entropy of the system, (T m ) i is the metal melting point of element i, and R is the gas constant. Table 2 lists the characteristic parameters of the elements of Co 32 Cr 28 Ni 32 . 94 Al 4 . 06 Ti 3 , and Table 3 lists the mixing enthalpy values of the atom pairs of the HEA elements [18].
According to the calculations, the difference in the atomic size (δ) of the HEA was 4.26%. The disorder of the system (Ω) of the HEA was 3.91, and the VEC of the HEA was 8.1. Therefore, the HEA considered here should be an FCC solid-solution structure in theory.  The lattice constants of the five elements are listed in Table 4. The lattice constants of alloys can be calculated using the disorder principle [21]: where c i is the atomic percentage of element i, and a i is the lattice constants of element i. According to the calculations, the lattice constant of Co 32 Cr 28 Ni 32 . 94 Al 4 . 06 Ti 3 was 0.304 nm, which was consistent with the lattice constant obtained as per XRD analysis (Table 4). The theoretical density of the alloy was calculated using the following formula [22]: where c i is the atomic percentage of element i, A i is the atomic weight of element i, and ρ i is the density of element i. From the calculation results, we noted that the theoretical density (7.85 g/cm 3 ) of the HEA was almost identical to the measured value (7.84 g/cm 3 ). It was also found that the HEA results were consistent with the rule of mixtures upon comparing the lattice constant and theoretical density of the alloy, which was a disordered FCC solid-solution structure. Figure 3 shows a light microscopy image of the metallographic structure of Co 32 Cr 28 Ni 32 . 94 Al 4 . 06 Ti 3 . As shown in Figure 3, the alloy had a uniform, single-phase, equiaxial crystal structure, and the grain size was about 163 µm as measured using the transection method.  Figure 4 shows the tensile stress-strain curve of Co 32 Cr 28 Ni 32 . 94 Al 4 . 06 Ti 3 . As shown in Figure 4, the yield strength was 530 ± 6 MPa, the tensile strength was 985 ± 7 MPa, and the elongation was 37.16 ± 0.17%. Compared with the CoCrNi alloy, the yield strength and the tensile strength increased by 103% and 14%, while the elongation decreased by 7%. Obviously, the increase in Al and Ti elements improved the tensile properties of the CoCrNi baselime alloy. Moreover, from Table 5, which compares the uniaxial tensile test results of similar alloys, it can be seen that the alloy affords better strength and plasticity. Moreover, the microhardness of the alloy was 313 HV. Therefore, Co 32 Cr 28 Ni 32 . 94 Al 4 . 06 Ti 3 exhibits satisfactory tensile mechanical properties and hardness.   [23] 450 700 18 CoCrFeNiW 0 . 4 [10] 525 970 11 Al 3 CoCrFeNiTi [11] 115 152 26 CoCrFeMnNi [24] 468 590 30 N18 Zircaloy [25] 390 420 38 N36 Zircaloy [25] 310 520 27 The tensile properties of the Co 32 Cr 28 Ni 32 . 94 Al 4 . 06 Ti 3 studied in this paper are obviously improved compared with N18 and N36 Zircaloy currently used in the nuclear industry, as shown in Table 5. Therefore, the alloy studied in this paper is could be used in the nuclear industry.
Generally, the main strengthening mechanism of a single disordered FCC solidsolution structure is solid-solution strengthening, which mainly originates from the interaction between solute atoms [26]. Figure 5 shows an SEM micrograph of the fractured tensile specimen of Co 32 Cr 28 Ni 32 . 94 Al 4 . 06 Ti 3 . A large number of dimples and holes were observed in the fracture. The observed dimples were the traces left on the fracture after micropore nucleation and aggregation, which was the characteristic of micropore aggregation fractures. The results provide a reference for the study of microstructure and properties of CoCrNi HEAs. In order to fully explain the comprehensive mechanical properties of the HEA, further studies on CoCrNiAlTi HEAs are being carried out, such as the effects of different atomic ratios of major elements on the creep, fatigue properties, and irradiation resistance of CoCrNiAlTi HEAs.